Chapter Fourteen: ACIDS AND BASES

1

Contents

p622

14-1 The Nature of Acids and Bases

p623

Arrheniuis postulates that acids produce hydrogen ios in aqueous solution, while bases produce hydroxide ions. Danish chemist Johannes Bronsted and the English chemist Thomas Lowry In terms of the Bronsted-Lowry model, an acids is a proton (H+) donor, and a base is a proton acceptor. Arrhenius: Acids produce H+ in solution, bases produce OHion. Brønsted-Lowry:Acids are H+ donors, bases are proton acceptors. HCl + H2O  Cl+ H3O+ HCl + H2O  Cl+ H3O+ acid base

3

Models of Acids and Bases 

Arrhenius: Acids produce H+ in solution, bases produce OH-ion.



Brønsted-Lowry: Acids are H+ donors, bases are proton acceptors. HCl + H2O → Cl-+ H3O+ acid base

4

Conjugate acid-base pair

p624

5

The equilibrium expression for Ka HA(aq) + H2O

H3O+(aq) + A-(aq)

acid

onjugate cid

base

p625

conjugate base

6

Brønsted-Lowry Reaction

7

Ex 14.1 Acid Dissociation (lonization) Reactions

P625

Write the simple dissociation (ionization) reaction (omitting water) for each of the following acids. 

a. Hydrochloric acid (HCl)



b. Acetic acid (HC2H3O2)



c. The ammonium ion (NH4+)



d. The anilinium ion (C6H5NH3+)



e. The hydrated aluminum(III) ion [Al(H2O)6]3+

p625

The Bronsted- Lowry model is not limited to aqeous solutions

10

p625

In this reaction is donated by the hydrogen chloride to the ammonia, as shown these Lewis structure.

11

14-2 Acid Strength

p626

The strength of an acid is defined by the equilibrium position of its dissociation (ionization) reaction. Before dissociation

After dissociation at equilibrium

p626

Figure 14.5

The relationship of acid strength and conjugate base strength for the ionization reaction.

13

A strong acid is virtually 100 %

p627

dissociated in water.

14

Acid Strength

p627

p627

Figure 14.6

(a) A strong acid HA is completely ionized in water. (b) A weak acid HB exists mostly as undissociated HB molecules in water. Note that the water molecules are not sown in this figure.

16

Acid Ionization Equilibrium

17

Self-Ionization of Water

18

ct a e R

HA(aq) + H2O acid base

H3O+(aq) + A-(aq) conjugate conjugate acid base

What is the equilibrium constant expression for an acid acting in water? 19

Think!



How good is Cl-(aq) as a base?



Is A-(aq) a good base?

20

Think! 



H2O(aq) + H2O acid base

H3O+(aq) + OH-(aq) conjugate conjugate acid base

At 25 C, Kw = 1.0 x 10-14

21

Values of Ka

p628

Appendix 5.1 contains a table of Ka values. Organic acids: carboxyl group

p628

P628

Ex 14.2 Relative Base Strength Using Table 14.2, arrange the following species according to their strengths as bases: H2O, F-, Cl-, NO2-, and CN-. Solution

24

Water as an Acid and a Base

p629

25

Kw

p629

26

p630

There are three possible situations:

27

P630

Ex 14.3 Calculating [H+] and [OH-] 

Calculate [H+] or [OH-] as required for each of the following solutions at 25℃, and state whether the solution is neutral, acid, or basic.



a. 1.0 × 10 -5 M OH-



b. 1.0 × 10-7 M OH-



c. 10.0 M H+ 28

p630 Solution:

Ex 14.4 Autoionization of Water

P630

At 60 ℃, the value of Kw is 1 × 10-13. a. Using Le Chatelier’ s principle, predict whether the reaction is exothermic or endothermic. 2 H 2O(l )  H 3O (aq) OH (aq)

b. Calculate [H+] and [OH-] in a neutral solution at 60 ℃. 30

p631 Solution:

14.3 The pH Scale

p631

32

The pH Scale

33

P631

Ex 14.5 Calculating pH and pOH Calculate pH and pOH for each of the following solutions at 25 ℃ a. 1.0 × 10 -3 M OHb. 1.0 M OH-

34

p632 Solution:

(a)

(b)

p632

The pH scale and pH values of some common substances

Figure 14.8

36

P633

Ex 14.6 Calculating pH The pH of a sample of human blood was measured to be 7.41 at 25℃. Calculate pOH, [H+], and [OH-] for the sample.

37

p634

14-4 Calculating the pH of Strong Acid Solutions

Ex 14.7 pH of Strong Acids M HCl. a. Calculate the pH of 0.10 M HNO3. b. Calculate the pH of 1.0 × 10 Solution: (a) (b)

-10

p634

14-5 Calculating the pH of Weak Acid Solutions

p635

Ex 14.17 Calculating Ka from Percent Dissociation p643

Lactic acid is a waste product that accumulates in muscle tissue during exertion, leading to pain and a feeling of fatigue. In a 0.100 M aqueous solution, lactic acid is 3.7% dissociated . Calculate the value of Ka for this acid. Solution:

14.6 Bases

p644

42

Ex 14.12 The pH of Strong Bases

p645

Calculate the pH of a 5.0 x 102- M NaOH. Solution:

43

The lone pair is located on a nitrogen atom

p646

44

Kb

p647

Ex 14.13 The pH of Weak Bases I

p647

Calculates the pH for a 15.0 M NH3(Kb = 1.8 x 10-5). Solution:

14.7 Polyprotic Acids

p650

Polyprotic acids

p651

Ex14.15 The pH of a Polyprotic Acid

p651

Calculate the pH of a 5.0 M H3PO4 solution and the equilibrium concentrations of the species H3PO4, H2PO4-, HPO42-, and PO43-. Solution: a.

e. b.

c.

f. d.

Acid-Base Properties of Salts

Cation neutral neutral

conjugate acid of weak base conjugate acid of weak base

Anion neutral conjugate base of weak acid neutral

conjugate base of weak acid

Acidic or Basic neutral basic

Example NaCl NaF

acidic

NH4Cl

depends on Ka & K b values

Al2(SO4)3 50

14-8 Acid-Base Properties of Salts Salts That Produce Neutral Solutions KCl, NaCl, NaNO3, and KNO3 are neutral.

Salts That produce Basic Solutions The pH of the solution will be determined by the conjugate anions of weak acids, such as CN-, acetate ion C2H3O2-.

p655

Ex 14.18 Salts as Weak Bases Calculating the pH of a 0.30 M NaF solution. The Ka value for HF is 7.2 x 10-4. Solution:

p656

Base Strength in Aqueous Solutions Relative base strength: OH- > CN- > H2O Salts That Produce Acidic Solutions Table 14.5 Qualitative Prediction of pH of Salt Solutions of Salts for Which Both Cation and Anion Have Acidic or basic properties

p657

Various Types of Salts

54

14-9 The Effect of Structure on Acid-Base Properties Table 14.7 Bond Strengths and Acid Strengths for Hydrogen Halides

p661

The structure effect



Two factors for acidity in binary compounds: 

Bond Polarity (high is good)



Bond Strength (low is good)

p661

p662

The Effect of Structure on Acid-Base Properties The effect of number of attracted oxygens on the O-H bond in a series of chlorine oxyacids.

Figure 14.11

p662

14-10 Acid-Base Properties of Oxides

Oxides 

Acidic Oxides (Acid Anhydrides): • O X

bond is strong and covalent.

SO2, NO2, CrO3  Basic Oxides (Basic Anhydrides): • O X

bond is ionic.

K2O, CaO 59

14-11 The Lewis Acid-Base Model

p663

Three Models for Acids and Bases

60

Lewis model

p664

61

Lewis acid-Lewis base reaction Hydration of a metal ion. 3+

Al3+ + 6 O

H

H Al H

O H

Figure 14.12 Reaction of BF3 with NH3

6

p664

Al3+

ion accepts one electron pair from each of six water molecules to form Al(H2O)63+.

p665

14-12 Strategy for Solving Acid-Base p666 Problems: A Summary